The present invention is directed to a method of calibrating a well pumpoff controller using a non-calibrated load transducer producing a signal output which may drift with changes in temperature in which the pumpoff controller may be calibrated to offset the signal drift of the load cell.
It is well known as described in U.S. Pat. No. 4,286,295 to utilize a well pumpoff controller for pumping liquid from a well by measuring the load and position of a pump rod and pumping the well for a preset pump time. If the load on the down stroke is greater than the pumpoff control set point for a preselected number of consecutive strokes, the controller will shut the pump down for a predetermined downtime and thereafter restart the operation cycle.
Load cells of various types and designs have been in use in pumpoff controllers. Polished rod mounted type load transducers provide good accuracy and calibration. However, polished rod mounted load transducers are expensive and are subject to damage during operation. Beam mounted load transducers are simpler, less expensive, and have long operational life with low maintenance. However, such beam mounted transducers produce a relative signal output rather than a calibrated one and the output signal may drift with changes in ambient temperatures. That is, a beam mounted load cell is welded to the well walking beam during installation and typically has a non-zero output signal. Also, because of solar heating effects to the walking beam, the signal output of the tranducer drifts with temperature changes of the walking beam.
It is known as disclosed in U.S. Pat. No. 4,583,915 to adjust the minimum load measurement to overcome thermal drift. However, this correction does not provide the desired result under many well conditions.
The present invention is directed to a method of calibrating a pumpoff controller by using the average load for providing an offset signal to the load measurement for obtaining a near calibrated signal while using a simple, inexpensive non-calibrated load transducer.